Stage 2: Synthesis of 4,6-difluoro-1H-indole-2,3-dione
The product of stage 1, 2-hydroxyimino-N-(3,5-difluorophenyl)-acetamide (21.1 g, 105 mmol) was slowly added to concentrated sulfuric acid (115 ml) preheated to 50-60°C. After maintaining this temperature and stirring for 15 minutes, the reaction mixture was warmed up to 100°C and stirring was continued for 30 minutes. After completion of the reaction, the mixture was cooled to room temperature and slowly poured into ice (1.2 kg). The precipitated solid was collected by filtration, washed to neutrality with plenty of water and finally dried in a desiccator using phosphorus pentoxide as desiccant. The yellow solid product 4,6-difluoro-1H-indole-2,3-dione was obtained 18.4 g in 95% yield.
1H-NMR (DMSO-d6) data: δ 6.60 (1H, dd, J = 8.8 and 2.0 Hz); 6.84 (1H, dt, J = 9.8 and 2.0 Hz); 11.20 (1H, br s).
